In general, an active-matrix display apparatus has a multiplicity of pixels arranged in matrix and displays an image by controlling the intensity of light pixel by pixel in accordance with image signals given. When, for example, liquid crystal is used as an electro-optic substance, the transmittance of each pixel varies in accordance with the voltage applied to the pixel. The basic operation of an active-matrix image display apparatus employing an organic electroluminescence (EL) material as an electro-optic converting substance is the same as in the case where liquid crystal is used.
A liquid crystal display panel has pixels each functioning as a shutter and displays an image by turning on/off light from a back light with such a shutter, or a pixel. An organic EL display panel is a display panel of the self-luminescence type having a light-emitting device in each pixel. Such a self-luminescence type display panel has advantages over liquid crystal display panels, including higher image visibility, no need for a back light, and higher response speed.
The organic EL display panel controls the luminance of each light-emitting device (pixel) based on the amount of current. Thus, the organic EL display panel is largely different from the liquid crystal display panel in that its luminescent devices are of the current-driven type or the current-controlled type.
Like the liquid crystal display panel, the organic EL display panel can have any one of a simple-matrix configuration and an active-matrix configuration. Though the former configuration is simple in structure, it has a difficulty in realizing a large-scale and high-definition display panel. However, it is inexpensive. The latter configuration can realize a large-scale and high-definition display panel. However, it has problems of a technical difficulty in control and of a relatively high price. Presently, organic EL display panels of the active-matrix configuration are being developed intensively. Such an active-matrix EL panel controls electric current passing through the light-emitting device provided in each pixel by means of a thin film transistor (TFT) located inside the pixel.
An organic EL display panel of such an active-matrix configuration is disclosed in Japanese Patent Laid-Open Publication No. HEI 8-234683 for example. FIG. 62 shows an equivalent circuit of one pixel portion of this display panel. Pixel 216 comprises an EL device 215 as a light-emitting device, a first transistor 211a, a second transistor 211b, and a storage capacitor 219. Here, the EL device 215 is an organic electroluminescence (EL) device.
In the present description, a transistor for feeding (controlling) current to an EL device is referred to as a driving transistor, while a transistor operating as a switch like the transistor 211b in FIG. 62 referred to as a switching transistor.
EL device 215 has a rectification property in many cases and hence is called OLED (Organic Light-Emitting Diode) as the case may be. For this reason, the EL device 215 in FIG. 62 is regarded as an OLED and represented by the symbol of a diode.
In the example shown in FIG. 62, the source terminal (S) of p-channel transistor 211a is connected to Vdd (power source potential), while the cathode (negative electrode) of the EL device 215 connected to ground potential (Vk). On the other hand, the anode (positive electrode) is connected to the drain terminal (D) of the transistor 211b. The gate terminal of the p-channel transistor 211b is connected to a gate signal line 217a, the source terminal connected to a source signal line 218, and the drain terminal connected to the storage capacitor 219 and the gate terminal (G) of the transistor 211a. 
In order to operate the pixel 216, first, the source signal line 218 is applied with an image signal indicative of luminance information with the gate signal line 217a turned into a selected state. Then, the transistor 211b becomes conducting and the storage capacitor 219 is charged or discharged, so that the gate potential of the transistor 211a becomes equal to the potential of the image signal. When the gate signal line 217a is turned into an unselected state, the transistor 211a is turned off, so that the transistor 211a is electrically disconnected from the source signal line 218. However, the gate potential of the transistor 211a is stably maintained by means of the storage capacitor 219. The current passing through the EL device 215 via the transistor 211a comes to assume a value corresponding to voltage Vgs across the gate and the source terminals of the transistor 11a, with the result that the EL device 215 keeps on emitting light at a luminance corresponding to the amount of current fed thereto through the transistor 211a. 
As described above, according to the prior art configuration shown in FIG. 62, one pixel comprises one selecting transistor (switching device) and one driving transistor. Another prior art configuration is disclosed in Japanese Patent Laid-Open Publication No. HEI 11-327637 for example. This publication describes an embodiment in which a pixel comprises a current mirror circuit.
Meanwhile, the organic EL display panel is usually manufactured using a low temperature polysilicon transistor array. Since organic EL devices emit light based on current, the organic EL display panel involves a problem that display irregularities occur if there are variations in transistor characteristics.
Further, a conventional EL display panel cannot sufficiently charge/discharge the parasitic capacitance which is present in the source signal line 18. For this reason there arises a problem that in some cases a desired current cannot be fed to pixel 16.